This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
London exists where it does because of one stretch of river geography: the Thames is tidal from Teddington Lock in the southwest all the way to the sea, and the Romans needed the furthest point downstream where that tidal river could still be bridged reliably. Too far upstream and a settlement loses the tidal port that lets seagoing ships reach it; too far downstream and the river is too wide and too fast to bridge with the technology available. London Bridge's original crossing sits close to the practical answer to that problem, and the city grew outward from a river crossing rather than from any other kind of natural advantage.
What the tidal river gave London afterward was a working harbor inside a city, not just beside one. Ships could ride the tide in from the North Sea, through the Thames Estuary, and dock in the Pool of London on the same tide, load or unload, and ride the next tide back out, which is a large part of why London became the trading capital it did rather than remaining a fortified river crossing. The docks that lined the Thames through the nineteenth century, and the warehouses that still stand converted to flats along its banks today, are the physical residue of a city whose whole reason for existing was a specific, exploitable fact about where a tidal river could be crossed and where, just downstream of that crossing, it could still be sailed.
The Thames did not stop shaping London once the bridges multiplied and the railways arrived. Flood risk from the tidal river below Teddington remains real enough that London built the Thames Barrier across the river at Woolwich specifically to hold the sea out during storm surges, a modern, engineered answer to the same tidal geography that decided where the city would be founded in the first place.